Radon 222 is a radioactive gas formed by the decay of uranium. Although an underground deposit of uranium is relatively immobile, the radon gas it produces gradually rises to the surface. At or just beneath the surface, detectors can be positioned in a grid pattern to detect and count the alpha particles emitted when radon 222 decays. In this way, a map of the area can be produced which hopefully shows the location of the underground uranium deposit.
Another naturally-occurring radioactive element is thorium. Thorium decays to the radioactive gas radon 220 (commonly known as "thoron"), which also emits alpha particles when it decays. Therefore, the detection of large quanitities of alpha particles from radon gas may indicate that thorium is underground, not uranium. At current prices, thorium is not worth mining, but, until now, a simple, low-cost method of distinguishing between alpha particles from radon 220 and alpha particles from radon 222 was not available.